Hydroxide based decomposition pathways of alkyltrimethylammonium cations

A systematic study that altered the number of β-hydrogen atoms susceptible to Hofmann elimination and introduced increased steric hindrance of substituted (ethyl, n-propyl, isobutyl, and neopentyl) alkyltrimethylammonium cations was performed. The mechanism of the thermal decomposition of these four ammonium cations in deuteroxide form was studied using evolved gas analysis (EGA) because of their potential importance in alkaline membrane fuel cells or electrolyzers. The products of the decomposition reactions are in many cases the expected Hofmann elimination products (trimethylamine and olefins), however, as the number of β-hydrogen atoms decrease or they become more sterically encumbered (from the addition of adjacent methyl groups), nucleophilic attack of hydroxide on the methyl groups increases in relative importance. The use of deuterated water and deuteroxide in our study shows that deprotonation of the tetraalkylammonium ions establishes a rapid equilibrium between the nitrogen ylide species that is formed by methyl group deprotonation and water that scrambles deuterium into the methyl groups of the amine. The results of this work show that at high temperature and low water content tetraalkylammonium hydroxide salts are relatively unstable in membranes.

► Systematic study of the decomposition of alkyltrimethylammonium cations.
► Controlled heating conditions to monitor degradation products.
► Degradation products analyzed by evolved gas analysis.
► Nitrogen ylide formation observed along with Hofmann and nucleophilic SN2 attack degradation products.
► Cation stability has implications for alkaline fuel cell membranes.